Bactrim (Trimethoprim-Sulfamethoxazole) Should Be Avoided During the First Trimester of Pregnancy
Trimethoprim-sulfamethoxazole is not safe for use during the first trimester of pregnancy and should be avoided due to associations with congenital malformations, particularly neural tube defects, cardiovascular abnormalities, oral clefts, and increased risk of spontaneous abortion.
Evidence from Guidelines and FDA Labeling
The most authoritative sources consistently recommend against first-trimester use:
The European Respiratory Society/Thoracic Society of Australia and New Zealand (2020) explicitly states to "Avoid" co-trimoxazole during the first trimester (T1), citing associations with hyperbilirubinemia of the neonate and fetal hemolytic anemia when used at delivery 1.
The FDA drug label for Bactrim warns that some retrospective epidemiologic studies suggest an association between first-trimester exposure and increased risk of congenital malformations, particularly neural tube defects, cardiovascular abnormalities, urinary tract defects, oral clefts, and club foot 2.
The CDC guidelines (2013) for Q fever treatment classify trimethoprim-sulfamethoxazole as category C and note that its use during pregnancy might increase the risk for congenital abnormalities (primarily urinary tract and cardiovascular abnormalities) because of antifolate effects 3.
Specific Malformation Risks
The most recent high-quality research demonstrates concerning associations:
A 2025 cohort study of 71,604 pregnancies found that first-trimester TMP-SMX exposure was associated with a 35% increased risk of any congenital malformation (RR 1.35,95% CI 1.04-1.75) compared to β-lactam antibiotics 4.
This same study identified elevated risks for severe cardiac malformations (RR 2.09), other cardiac malformations (RR 1.52), and cleft lip and palate (RR 3.23) with TMP-SMX exposure 4.
A 2016 case-control study among women with urinary tract infections found TMP-SMX use was associated with esophageal atresia (adjusted OR 5.31) and diaphragmatic hernia (adjusted OR 5.09) 5.
Pregnancy Loss Risk
Beyond structural malformations, TMP-SMX poses risks for pregnancy viability:
A 2018 nested case-control study of 77,429 pregnancies found TMP-SMX exposure was associated with nearly a 3-fold increased risk of spontaneous abortion (adjusted OR 2.94,95% CI 1.89-4.57) 6.
A 2020 systematic review identified an association between TMP-SMX and spontaneous abortion (OR 3.5,95% CI 2.3-5.6), preterm birth (OR 1.5), and small for gestational age (OR 1.6) 7.
Mechanism of Harm
The teratogenic effects are biologically plausible:
Both trimethoprim and sulfamethoxazole interfere with folic acid metabolism, which is critical for neural tube closure and other embryonic development processes during the first trimester 8, 3.
The FDA label explicitly states that "sulfamethoxazole and trimethoprim may interfere with folic acid metabolism" and recommends use only if potential benefit justifies potential risk to the fetus 2.
Clinical Alternatives
For infections requiring treatment during the first trimester:
β-lactam antibiotics (penicillins and cephalosporins) are the safest alternatives and should be used preferentially 1, 9, 4, 10.
Nitrofurantoin is considered "possibly safe" during the first trimester and represents another alternative, though it showed a non-significant trend toward increased malformation risk (RR 1.12) in the 2025 study 1, 4.
Special Circumstances
There are rare exceptions where TMP-SMX may be considered:
For pregnant women with acute Q fever, long-term TMP-SMX treatment throughout pregnancy has been shown to significantly decrease adverse fetal outcomes (from 81% untreated to 40% treated), though complications still occur 3.
In this specific scenario, the CDC recommends concomitant folic acid supplementation (5 mg/day) if TMP-SMX must be used during the first trimester 1, 3.
Key Clinical Pitfalls
Do not assume that because TMP-SMX is commonly prescribed, it is safe in pregnancy—the evidence clearly demonstrates first-trimester risks 2, 4.
Avoid the misconception that folic acid supplementation alone adequately mitigates the teratogenic risk—while recommended if TMP-SMX must be used, it does not eliminate the elevated malformation risk 1, 3.
Do not continue TMP-SMX in women of reproductive age without ensuring effective contraception or discussing pregnancy planning 2.